Vehicles are operated by power generated from the process of burning fuel. A vehicle has an engine mounted at a front or a rear part thereof. A vehicle also has a fuel tank for storing fuel necessary for operation of the engine. A fuel line is formed between the fuel tank and the engine for feeding the fuel pumped from the fuel tank to the engine. The fuel line includes a fuel supply line to allow fuel to flow from the fuel tank side of the engine to the engine, and a fuel return line for returning residual fuel to the fuel tank when there is remaining fuel after the fuel has been supplied to the engine.
Meanwhile, a vehicle engine is manufactured in an engine assembly line separate from a finished vehicle assembly line, and goes through an appropriate pre-test process. Once the engine passes a standard level, it is transferred to a finished vehicle assembly line to be mounted in a vehicle.
The engine pre-test process tests whether an engine is operational. In order to start the engine for pre-testing, a fuel filter mounted at one side of the engine is forcibly fed with an appropriate amount of fuel necessary for operation during the pre-testing period.
However, there is a problem in that, when an engine that has passed the pre-testing is transferred to a finished vehicle assembly line to be mounted in a vehicle, and a cap blocking a connector of the fuel filter is removed, fuel remaining in the fuel filter flows out of the filter, resulting in contamination of surroundings and waste of fuel. This problem occurs not only in a finished vehicle assembly line but also in engine maintenance.